The present invention relates generally to producing high power output from compact electrical generators. More particularly, the invention relates to cooling such generators and mitigation of windage losses that may result from such cooling.
There is an increasing need for high power generators in the 500 kilowatts (kW) to 1 megawatt (MW) range for hybrid-electric propulsion and directed energy weapons. At the same time, these applications require low weight and volume. Small and light-weight generators must be operated at high rotational speeds in order to produce high power outputs. High amounts of heat are produced during operation of such generators. Oil cooling is typically employed to maintain operating temperatures of such generators at a tolerable level.
Oil-based cooling systems are advantageously compact and efficient, but there has heretofore been a practical limit to their effective application. At rotational speeds of about 12,000 revolutions per minute (rpm) or more, a generator may experience high windage friction forces that develop when oil enters a gap between a rotor and a stator of the generator. As the rotor rotates at a high speed, windage friction may heat the oil in the gap beyond its temperature limit if not replenished with an adequate amount of lower temperature oil. This constant oil bath may have the counterproductive effect of producing higher rotating friction or windage reducing the overall efficiency of the generator. In that context, heat-reducing benefits of oil-based cooling may be offset by heat production resulting from windage friction arising from a presence of oil in the air gap. Undesirable windage friction may also arise if cooling oil is sprayed in the generator and the oil spray is allowed to contact exterior surfaces of the rotor.
As can be seen, there is a need for generator cooling system that will retain its effectiveness at high rotational speeds. More particularly, there is a need for such a system that may eliminate windage friction resulting from cooling oil contact with exterior surfaces of a rotor and/or presence of oil in an air gap between a rotor and a stator of the generator.